As a conventional location detecting system which detects the location of a vehicle being travelling on an arbitrary place of road traffic network, there has been proposed a system which uses the principle of dead reckoning and comprises a distance sensor, a heading or direction sensor and a processing unit for giving necessary processes to output signals from the distance and heading sensors. In the dead reckoning, a data regarding the present location of a vehicle is obtained by integrating the amount of change in distance and heading. However, there is the drawback that the distance and heading errors that are inevitably inherent in the distance and heading sensors are accumulated with vehicle travel, and the errors in the present location data obtained by the distance and heading sensors are also accumulated.
In order to overcome the drawback described above, map matching methods have been proposed in U.S. Pat. No. 3,789,198, Japanese patent laid-open publication No. 58-99715, Japanese patent laid-open publication No. 58-113711, "LANDFALL A HIGH-RESOLUTION AUTOMATIC VEHICLE-LOCATION SYSTEM", D. KING, GEC Journal of Science & Technology, Vol. 45, No. 1, 1987, and "Experimental Study to Automotive Travel Electronic Technology", Society of Automotive Travel Electronic Technology, March, 1982. In the map matching methods, the present location data obtained in accordance with the aforementioned dead reckoning is compared with a previously stored road traffic network data, the departed amount of the present location data from the road data is computed as an accumulated error, and the present location data is corrected by the accumulated error and matched with the road data.
More specifically, (1) in the location detecting method described in the U.S. Pat. No. 3,789,198, the present location data of a vehicle is calculated upon the travel distance data obtained by a distance sensor and the travel heading data obtained by a heading sensor. The calculated present location data is compared with a previously stored road location data. If the difference between the present location data and the road location data is within a predetermined threshold value, the present location data is corrected so as to correspond with the nearest road, and on the other hand, if the difference between the both data exceeds the predetermined threshold value, no correction is made, in order that the correction can be made with high precision and the present location data can be displayed accurately.
(2) In the location detecting method described in the Japanese patent laid-open publication No. 58-99715, to display an accurate present location, the present location data of a vehicle is updated at every predetermined distance by calculating the amount of distance change with respect to the coordinate directions of road map, and when the present location data is departed from the road, the location data corresponding to the nearest road is merely determined as a present location data.
(3) In the vehicle location detecting method described in the Japanese patent laid-open publication No. 58-113711, to display an accurate present location, errors arising from a travel heading sensor for detecting a heading or direction of vehicle travel with respect to the earth's magnetic axis are corrected by comparing the radius of curvature obtained upon the travel heading with the radius of the road.
(4) In the vehicle location detecting method described in the "LANDFALL A HIGH-RESOLUTION AUTOMATIC VEHICLE-LOCATION SYSTEM", road traffic network is grouped into non-branch parts and a plurality of typical branch parts, and when the vehicle is travelling from any one of the branch parts to other branch part, the distance between the two branch parts is calculated upon the signal from a distance sensor, and when it was determined that the vehicle reached the branch part, the branch exit to which the vehicle was directed is determined by detecting the travel heading change thereafter by a heading sensor, in order to accurately display the location of a vehicle as a location on the road.
(5) In the location detecting method described in the "Experimental Study to Automotive Travel Electronic Technology", in the case a vehicle is travelling on a road other than an intersection, the location of the vehicle is detected as a location on the road, and in the case a vehicle travels an intersection, by detecting a greater change in heading, the location of the vehicle is corrected to the intersection. However, in the case a vehicle travels an intersection wherein a plurality of roads are branched, the location of the vehicle is not corrected and detected by a travel heading data and a travel distance data (dead reckoning), and thereafter no correction is made until branched roads are determined as a unitary road.
The location detecting methods described above can display accurately the present location of a vehicle by correcting an increase in accumulated errors, in the case road traffic network is relatively simple. However, when trucks and the like approach and in the areas where buildings, railway crossings and the like are present, a heading sensor is subjected to errors because of the influence of the external magnetic field, and as a result the accumulated errors resulting from the heading sensor which causes greater errors as compared with a distance sensor are to be included in the present location data of a vehicle as an error which cannot be corrected. If, therefore, the present location data is merely corrected by corresponding with the nearest road, there will be the possibility of displaying a road entirely different from an actual road as a present location. On the contrary, if, as in the case of the aforementioned method (5), the present location is detected only by dead reckoning without correcting until a unitary road is determined, there is the problem that a unitary road cannot be determined eternally, and ultimately distance errors are accumulated.
On the other hand, in a distance sensor for detecting a distance that a vehicle travels, the accumulated errors caused by the distance sensor occasionally exceed an allowable limit value as the distance sensor is subjected to the influence of air pressure and like of the tire. Therefore, the same problem as the above case is also involved in the distance sensor.
And, even if the occurrence probability of the problems described above is in a low condition, there is the problem that, once accumulated errors exceed an allowable limit value, the vehicle location data detected thereafter becomes entirely inaccurate as it is detected on the basis of an inaccurate present location data, and therefore the location detecting systems of the type described above are not suitable for practical use.
In order to eliminate the aforementioned problems, the inventors of this application have proposed a location detecting method wherein an estimate location is obtained by an integrated data obtained by integrating the change in travel distance and travel heading at every predetermined interval, an error amount of the estimate location is obtained by the integrated data and an error of road map, the estimate location is registered as a self-location by corresponding with all roads located within a range of the error amount centering on the estimate location, a correlation coefficient corresponding to each road of the registered estimate locations is computed, a correlation coefficient showing the minimum error with respect to the road is selected, and an estimate location corresponding to the selected correlation coefficient is outputted as a present location.
In this location detecting method, all of the roads located within a range of the error amount corresponding to the estimate location are retrieved, the correlation coefficient between the road and the estimate location is computed, and the location of the road corresponding to the correlation coefficient having the minimum error can be outputted as a present location, thereby increasing an accuracy in the detection of position.
However, a subject road cannot be detected, in the case that a road map is in error, that a vehicle travels a large or complicated intersection, that a road map is partly omitted, that a vehicle travels a road which is not shown in a road map, for temporary reasons such as construction, traffic regulation and the like, or that a travel distance or travel heading sensor is damaged and therefore the sensor error became large. Even if a subject road can be detected, the correlation between the estimate location and road map cannot be detected as the degree of correlation becomes very small, or a road that does not coincide with an actual route of a vehicle is detected as a present location. As a result, the present location is detected only by dead reckoning, and therefore the total location error continues to increase because of sensor errors, and ultimately the present location must be manually corrected to correspond with the actual route of a vehicle by an operator.
Accordingly, it is an object of the present invention to provide an improved location detecting method which is capable of detecting an accurate present location of a vehicle without being subjected to the influence of the complexity of roads, errors in road map, a part omission in road map, sensor errors and the like.